The invention relates to switches for use in tools, such as downhole tools in wellbores.
In completing a well, different types of equipment and devices are run into the well. For example, a perforating gun string can be lowered into a wellbore proximal a formation that contains producible fluids. The perforating string is fired to create openings in surrounding casing as well as to extend perforations into the formation to establish production of fluids. Other completion devices that may be run into a wellbore include packers, valves, and other devices.
Electrical activation devices may be used to activate such completion devices, such as to fire a perforating gun, to set a packer, or to open or close a valve. Such electrical activation devices typically include switches that may be triggered to a closed position to electrically couple two components. In wellbore applications, the most common type of switch is made from a gas discharge tube that is either a triggered-type or over-voltage type switch. A triggered-type switch requires an external stimulus to close the switch or to activate it. An over-voltage switch is activated whenever the voltage level on one side of the switch exceeds a threshold value.
Conventional switches are constructed using a gas tube having an electrode on each end. In order to make the switch conduct, either a trigger voltage must be applied to a third internal grid or anode, or the switch is forced into conduction as a result of an over-voltage condition. The over-voltage switch, once manufactured, cannot be made to trigger at less than a preset voltage. It would be desirable to be able to trigger an over-voltage switch at a selectable lower voltage in order to perform margin testing on the system.
Further, the typical gas tube discharge switch is arranged in a tubular geometry, which is not conducive to achieving a switch having a low inductance (and thus low triggering voltage). Also, the tubular shape of a gas tube does not allow convenient reduction of the overall size of a switch. Additionally, it may be difficult to integrate the gas tube switch with other components.
Another type of switch includes an explosive shock switch. The shock switch is constructed using a flat flexible cable having a top conductor layer, a center insulator layer (made of KAPTON(copyright) for example), and a bottom conductor layer. A small explosive is detonated on the top layer causing the KAPTON(copyright) insulator layer to form a conductive ionization path between the two conductor layers. One variation of this is a xe2x80x9cthumb-tackxe2x80x9d switch in which a sharp metal pin is used to punch through the insulator layer to electrically connect the top conductor layer to the bottom conductor layer.
The explosive shock switch offers a low inductance switch but an explosive pellet must ignite to trigger the switch. The thumb tack switch is similar to the explosive switch but it may be relatively difficult to actuate. Thus, a need continues to exist for switches having improved reliability and triggering characteristics.
In general, according to one embodiment, a switch includes first and second conductors and an insulator electrically isolating the conductors. A device is responsive to an applied voltage to generate a plasma to perforate through the insulator to create an electrically conductive path between the first and second conductors.
Other features and embodiments will become apparent from the following description and from the claims.